This invention relates to a device for insuring the tight seal of closed space filled with a gas at high pressure and more particularly insuring a tight seal where there is an outlet rod through one wall forming the closed space such as in Stirling engines.
In mechanical apparatus, "closed" spaces are frequently encountered in which a vacuum has been created or which has been filled with a fluid having considerable differences in pressure with respect to medium outside the closed space. Such a space may be form from an assembleage of pieces or members which may have openings which may be sealed by doors, stoppers or like members. In the present state of the art, absolute tightness between these pieces, which are static with respect to one another, may be obtained due to seals of various types, whatever the pressure variation and nature of the fluid.
It is a different matter when the space comprises a mechanical outlet, i.e. an opening allowing the passage of a rod, piston or transmission shaft animated by a movement of rotation or of translation with respect to the space. In such a dynamic system, if the pressure variation is other than slight or if one or more of the fluids either in or outside the space is not highly viscous (i.e. a light gas), the seals to properly work must be so tight as to exert a pressure against the mobile faces of the joint with a risk of blocking the moving rod or shaft or alternating causing rapid deterioration of the seal may be used, but, if there is a pressure variation greater than a few tens of bars or if the fluid is not very viscous, such as a light gas, such a considerable tightening force must be exerted between the mobile faces of the joint that there is a risk of blocking the transmission shaft or causing rapid deterioration of the seal.
In the case of a transmission shaft in rotation, the best compromise is obtained by a mechanical packing in which the mobile faces in contact are in a plane perpendicular to the axis of rotation with a perfectly adjusted contact pressure. The leak of a fluid such as an oil at a pressure which may attain about a hundred bars is then extremely reduced. However, this arrangement, which already is not absolutely tight, is not applicable to a transmission shaft or rod in translation.
In this latter case, solutions are known which ensure tightness under certain conditions. Various types of supple, impermeable diaphragms are employed, constituted by thin metal materials or possibly reinforced elastomers and having the form of a surface of revolution; one of the edges of the surface is hermetically mounted on the transmission rod; the other edge of the surface is hermetically mounted on the opening of the space. The movements of translation in question are obviously reciprocating movements of limited stroke. The necessary movement of the diaphragm and its resistance to pressure are obtained by undulations judiciously disposed on a base form which may be a flat, circular or a diaphragm, conical or cylindrical bellows.
Another well-known advantageous arrangement is the rolling diaphragm or "sock" in which the diaphragm works solely in traction by rolling and abutting alternately, in the course of the movement, on the rod or on the bore made in the space. This arrangement is particularly advantageous to obtain a movement of considerable stroke with a seal of reduced dimensions.
However, these various hermetic solutions can be used in practice only for a pressure variation not exceeding a few tens of bars. In addition, the rolling diaphragm is destroyed by fatigue after a relatively low number of operational cycles, this giving it a very limited life if the reciprocating movement is of high frequency and with a considerable pressure variation.
A solution has already been found for the Stirling engine by a Dutch laboratory, and numerous Patents have been filed on the subject. This laboratory employs the rolling diaphragm described hereinabove, but applied to a liquid cushion; the diaphragm does not separate the working gas contained in the engine directly from the gaseous outside medium, with a pressure difference which should be able to attain 50 to 200 bars, but it separates it from a certain quantity of oil with a pressure variation of only about 5 bars; said quantity of oil is itself separated from the outside medium by a conventional dynamic seal as indicated hereinabove, supporting the considerable pressure variation. Arrangements are made for the volume offered to the oil in the course of the reciprocating movement to be constant. Under these conditions, a rolling diaphragm may, according to this laboratory, withstand two billion cycles. In addition, other precautions must be taken. To compensate for the oil volume variations due to the slight leakages of the dynamic seal, to the variations in temperature and to the absorption of gas by the oil, it is necessary to use a non-reinforced elastic rolling diaphragm and a pressure regulator must be installed, such as an adjusting valve, as well as a small high pressure oil pump which may be the dynamic seal operating as "pumping ring".